Photosensitizers and photoinitiators, used for the photoinduced free radical polymerization of monomers, are important in radiation curable coatings, bulk polymerization, and the graphic arts. Any improved efficiency in the utilization of incident light to photoinitiate polymerization allows for shorter exposure times or lower light intensities in the photocuring process, whether on a web, in solution or in graphic arts applications.
Carbonyl containing polymeric quaternary ammonium salts as photoinitiators of polymerization have been reported by Shibanov and Protsyuk [Russian Patent USSR No. 787,416, see Chemical Abstracts, 94, 122299r, 1981)]. Photoinitiated polymerization of methyl methacrylate with poly(methylisopropenyl ketone) was taught by Naito et al. [Kobunshi Ronbunshu 36, 777 (1979)]. Carlini et al. [Polymer, 24, 599 (1983)] cured acrylic compositions with benzophenone/dimethylaniline-substituted polymers.
The use of certain photosensitizers with onium salts for initiation of polymerization of ethylenically-unsaturated monomers is also well known in the art. This technique has found applications in printing, duplication, copying, and other imaging systems (see J. Kosar in Light Sensitive Systems: Chemistry and Application of Nonsilver Halide Photographic Procsses, Wiley, New York, 1965, pp 158-193). Aryliodonium salts have been previously described for use as photoinitiators in addition-polymerizable compositions. (See U.S. Pat. Nos. 3,729,313, 3,741,769, 3,808,006, 4,026, 705, 4,069,054, 4,250,053, 4,394,403 and 4,394,403; H. J. Timpe and H. Baumann, Wiss Z. Tech. Hochsch. Leuna-Merseburg, 26, 439 (1984); H. Baumann, B. Strehmel, H. J. Timpe and U. Lammel, J. Prakt. Chem., 326 (3), 415 (1984); and H. Baumann, U. Oertel and H. J. Timpe, Euro. Polym. J., 22 (4), 313 (Apr. 3, 1986).
Some monomeric Mannich bases used as free radical photoinitiators has been described in laid open Japanese patent application JA54-37182.
Whereas monomeric and Mannich bases derived from ketones are known, their use as photosensitizers as taught in this invention is novel.
Thus, what the background art has not taught but what this invention teaches is the general use of Mannich base monomers as photosensitizers for iodonium salts for use in the initiation of free radical polymerization of vinyl monomers. Moreover, these Mannich bases are unexpectedly more efficient as photosensitizers than are the conventional photosensitizers.